1. Field of The Invention
This invention relates to a process for preparing photochromic spiropyran compounds having a long chain at the N position of the compound and a halogen atom or atoms at the indoline ring thereof.
2. Description of The Prior Art
Photochromic materials are known as those materials which undergo reversible changes in color. One of typical photochromic materials includes spiropyran compounds. A number of spiropyran compounds have been studied and developed up to now. For instance, a spiropyran compound of the following formula (A) is irradiated with UV light with a wavelength of 340 nm, whereupon it is changed into merocyanine of the formula (B) assuming a red color. When visible light with a wavelength of 580 nm is again applied to the merocyanine, it is returned to the spiropyran (A). ##STR1##
This property of the photochromic materials is suitable for application, for example, as optical recording media. In order to obtain high recording sensitivity and stability, the recording medium should preferably have a uniform, ultra-thin recording film. One of techniques of forming such a ultrathin film under mild conditions is the Langmuir-Blodgett technique (hereinafter referred to simply as LB technique or method).
For the formation of the thin film by the LB method, spiropyran compounds should favorably have a hydrophilic site or part and a hydrophobic site or part in the molecule. The spiropyran compounds without modification are so low in hydrophobicity that a satisfactory LB film cannot be formed. In most cases, the LB films of spiropyran compounds are formed after introducing a long alkyl chain at the N position of an indoline ring so that the hydrophilicity and hydrophobicity of the compound are well balanced.
A number of spiropyran compounds have been proposed using various structural backbones and substituents in combination. Among them, spiropyran compounds having an indoline ring substituted with a halogen atom or atoms have absorptions at long wavelengths when converted to colored products. Accordingly, such spiropyran compounds are suitable for recording with semiconductor laser devices and are very stable, thus exhibiting good recording characteristics and good reliability.
For the formation of an LB film of spiropyran compounds which have an indoline ring substituted with a halogen atom or atoms, it is essential to introduce a long hydrophobic chain at the N position of the ring. A typical known process of preparing spiropyran compounds having a long hydrophobic chain at the N position is a process wherein an alkyl halide is reacted with indolenine as shown in the following reaction formula ##STR2##
Because the alkyl halide having a long alkyl chain is low in reactivity, the reaction with a halogen-free indolenine of the above formulae where Y=H proceeds but the reaction with a haloindolenine of the formulae, where Y=F, Cl, Br or I, does not proceed at all or may proceed only under very severe conditions. The reason for this is that the activity of the indolenine is lowered due to the electrophilic property of the halogen atom.
Thus, it has not been possible in an industrial sense to prepare halogenated spiropyran compounds having a long chain according to the known process. Thus, a thin film of a halogenated spiropyran has not been obtained by the LB method.